Dc-dc switching regulator with transconductance boosting

ABSTRACT

A switching regulator comprising a droop amplifier responsive to a reference voltage and a feedback voltage to generate a droop voltage. The droop amplifier includes a boost circuit configurable to increase a transconductance of the droop amplifier during an upward transition of the reference voltage. The switching regulator further includes a comparator responsive to the droop voltage and a current sense signal. The comparator is configured to initiate switching in the switching regulator.

TECHNICAL FIELD

Embodiments of the disclosure relate to DC-DC switching regulators.

BACKGROUND

DC-DC switching regulators with a finite load line are used to reduceoutput capacitance while meeting transient response. In such regulators,the load-line depends on the inductor current, not the actual loadcurrent. When the regulator transitions from a lower to a highervoltage, the output is lower than the target reference by load-line(droop) voltage that corresponds to the inductor current, since theoutput capacitor needs to be charged, at the end of the referencetransition. If the load transient is applied after the referencetransition, the output would go lower than its actual target voltage forthe given load current, thereby violating the specification of the powersupply and possibly resulting in malfunction of the load system.

SUMMARY

An example embodiment provides a switching regulator comprising a droopamplifier, responsive to a reference voltage and a feedback voltage, togenerate a droop voltage. The droop amplifier includes a boost circuitconfigurable to increase a transconductance of the droop amplifierduring an upward transition of the reference voltage. The switchingregulator further includes a comparator responsive to the droop voltageand a current sense signal. The comparator is configured to initiateswitching in the switching regulator.

Another example embodiment provides a switching regulator comprising adroop amplifier, responsive to a reference voltage and a feedbackvoltage, to generate a droop voltage; the droop amplifier comprising aboost circuit configurable to increase a transconductance of the droopamplifier during an upward transition of the reference voltage. Thedroop amplifier comprises a main resistor and an auxiliary resistor, theauxiliary resistor being coupled in parallel to the main resistor inresponse to the upward transition of the reference voltage through aswitch. The boost circuit comprises the auxiliary resistor coupled to aswitch, the switch being controlled to activate the boost circuit duringan upward transition of the reference voltage. The switching regulatorfurther includes a comparator responsive to the droop voltage and acurrent sense signal, the comparator being configured to initiateswitching in the switching regulator. The resistances of a main resistorand the auxiliary resistor are added in parallel upon activation of theboost circuit such that the transconductance of the droop amplifier isincreased thereby increasing gain of the droop amplifier.

Other aspects and example embodiments are provided in the Drawings andthe Detailed Description that follows.

BRIEF DESCRIPTION OF THE VIEWS OF DRAWINGS

FIG. 1 illustrates a schematic of a DC-DC switching regulator withdynamic output voltage control;

FIG. 2 illustrates a schematic of the control loop (inner current loopand outer voltage loop) with built-in droop, according to anotherembodiment;

FIG. 3 is a graph illustrating the characteristics of V_(DAC) andV_(FB);

FIG. 4 pictorially illustrates the DAC behavior during referencetransition;

FIG. 5 illustrates the improvement in the output voltage tracking in theswitching regulator having a boost circuit and offset compensationcircuit according to an embodiment; and

FIG. 6 illustrates the circuit diagram of the droop amplifier having theboost circuit and offset compensation circuit in a DC-DC switchingregulator.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 illustrates a DC-DC switching regulator with dynamic outputvoltage control. The DC-DC switching regulator, hereinafter, switchingregulator includes a dynamic output voltage control block having adigital to analog converter (DAC) (105) connected to a controlprotection block(110) which is then connected to a driver. The driver115 drives a set of transistors used as switches 122 (with theirrespective body diodes 120) in response to a driver signal or a controlsignal from the control protection block 110. A filter, having aninductor (125) and capacitor (130), is connected to the junction betweena drain and source of the transistors. A load 135 is connected to theoutput of the switching regulator. Several voltage signals are fed backfrom the switching regulator stages to the control protection blocknamely, V_(CSN) (current sense negative voltage), V_(SNS) (sensevoltage) V_(GNDSNS) (ground sense voltage), and V_(CSP) (current sensepositive voltage). The inductor 125 is connected to the capacitor 130 toprovide the output voltage of the switching regulator to the load 135.

Unlike a power supply with output voltage regulated to a fixed referenceall the time, in the dynamic dc-dc switching regulators, output voltageis continuously modulated depending on the load conditions to savepower, improve thermal performance, and extend battery life. The controlreference for the power supply is typically set by a DAC whose inputbits are either supplied through a serial or parallel bus from anintelligent host, which is tasked to provide control information to thepower supply as a function of load requirement. For such power supplies,it is desirable to have a finite load line, which means the outputvoltage is regulated to a lower voltage than the nominal reference whenload current increases. The advantage of having load-line or droop isthat when the load is released, output voltage overshoot is lower thanwhat would be seen if there is no load line in the power supplyimplementation. Alternatively, a smaller output capacitance can also beused in the power supply, thereby reducing cost and area footprint. Thekey to power supplies having load line is that the load works flawlesslywith a lower output voltage as compared to the reference with increasein current.

FIG. 2 illustrates a block diagram of the control loop (inner currentloop and outer voltage loop) with built-in droop. The block diagramincludes a DAC 205 that receives control bits for reference tracking.The DAC generates a reference voltage V_(DAC) that is connected to thedroop amplifier. The droop amplifier 210 is characterized as a drooptransconductance amplifier or in other words a droop amplifier with atransconductance of G_(MDRP). The droop amplifier 210 receives theV_(DAC) on a positive terminal and a feedback voltage V_(FB) on anegative terminal. The feedback voltage V_(FB) is generated from adifferential amplifier 225 which is the difference of voltage signalsV_(SNS) and GND_(SNS). A resistor (droop resistor) 215 is connected atthe output of the droop amplifier 210. The voltage droop can be setusing the resistor 215 which is an external resistor. A PWM comparator(comparator) 220 receives the output of the droop amplifier 210 and acurrent sense signal V_(ISNS) and generates the PWM output. The signalV_(ISNS) is amplified version of the differential signal between V_(CSP)and V_(CSN). The transconductance of the droop amplifier G_(MDRP) is adesigned parameter and provided to the end user so that resistance valueof the resistor 215 can be chosen for a given application. The PWMcomparator 220 compares V_(DROOP) and current sense signal V_(ISNS) andinitiates switching when V_(ISNS) goes below V_(DROOP).

Therefore, the mathematical models for the above operating conditionscan be expressed as follows:

${A_{ISNS}\left( {V_{CSP} - V_{CSN}} \right)} = {\left. {G_{MDRP}{R_{DRP}\left( {V_{DAC} - V_{FB}} \right)}}\Rightarrow V_{FB} \right. = {V_{DAC} - {\frac{A_{ISNS}}{G_{MDRP}R_{DRP}} \times I_{L}R_{SNS}}}}$

wherein A_(ISNS) is the gain of the current sense amplifer 230, V_(CSP)is current sense positive, V_(CSN) is current sense negative, G_(MDRP)is transconductance of the droop amplifier, R_(DRP) is droop resistance,V_(DAC) is reference voltage, V_(FB) is feedback voltage and I_(L) isaverage inductor current. Consequently, as inductor current increasesoutput voltage reduces as shown in FIG. 3. In FIG. 3, the X axisindicates average inductor current (I_(L)) and Y axis indicates thereference voltage (V_(DAC))/feedback voltage (V_(FB)).

FIG. 4 pictorially illustrates the DAC behavior during referencetransition. As a result of the in-built load line characteristics, whenthe converter (DAC) transitions from a lower reference to a highervoltage, although the actual load current can be very small, theinductor current is equal to the charging current of the outputcapacitance (of capacitor 130). The capacitance charging current dependson the slew-rate of the reference transition and the value of outputcapacitance. Therefore, at the time instance when reference transitionis complete, the output voltage is lower than the target reference by anamount that corresponds to the droop associated with capacitor chargingcurrent (indicated as 410). By way of equation, V_(FB1) will be thedifference between V_(DAC1) and V_(DRP), where V_(DAC1) is the targetreference and V_(FB1) is actual output voltage and V_(DRP) is the droopassociated with capacitor charging current. Depending on the droop atthat instance, the converter may take additional time to reach itstarget (shown as 405). Therefore, the host controller has to wait forlonger time before the power supply is ready to provide load demandwithout violating the desired load line specification.

To reduce the gain error completely, it is evident that the droopamplifier gain (G_(MDRP)R_(DRP)) needs to be a much higher than withwhich it operates under normal operating condition. Since the droopresistance (R_(DRP)) is set externally by the user, an embodimentincreases the transconductance of the droop amplifier (G_(MDRP)) onlyduring an upward transition of the reference voltage (V_(DAC)). Since itis impractical to boost the transconductance to a very high valuebecause of practical limitations and stability considerations, tocompensate for the error even after the gain enhancement, a fixed offsetis introduced in the controller reference path. This offset could be apositive value in the reference path or a negative value in the feedbackpath only during DAC up transition. As soon as the DAC up transition iscomplete, the offset is removed, boost circuit is disabled and theconverter operates with its nominal G_(MDRP) setting.

FIG. 5 illustrates the improvement in the output voltage tracking in theswitching regulator having a boost circuit and offset compensationcircuit. It can be seen from FIG. 5 that, V_(FB) with transconductanceenhancement and offset compensation (505) is closer to V_(DAC) in theDC-DC switching regulator thereby reducing the extra settling timerequired in case of the regulator without the boost circuit and offsetcompensation circuit.

FIG. 6 illustrates the circuit diagram of the droop amplifier 210 havingthe boost circuit 605 and offset compensation circuit 610 in a DC-DCswitching regulator during upward transition of the DAC referencevoltage according to an embodiment. The circuit illustrated in FIG. 6 isa circuit level implementation of the droop amplifier 210 of FIG. 2. Thedroop amplifier, that is configured to generate a droop voltage,includes the plurality of amplifiers 630 coupled to a plurality oftransistors, a main resistor 625 coupled between the two transistors, aplurality of current mirrors 615 (with a ratio of 1: K) and 620 (with aratio of 1:1). A boost circuit 605 is connected in parallel with themain resistor 625. The boost circuit includes an auxiliary resistor 605a connected to a switch 605 b where the auxiliary resistor 605 a isconnected in parallel to the main resistor 625 when activated. Theswitch 605 b is implemented using a transistor having a gate controlledby the upward transition of the reference voltage (REF_(UP)). The boostcircuit 605 is activated during the upward transition of the referencevoltage through the switch 605 b. The boost circuit 605 is activatedbased on an output of an up/down counter 635 wherein the up/down counterreceives a target DAC code and compares the target code against thecurrent DAC code such that during the upward transition of the referencevoltage the boost circuit is activated. Upon activation of the boostcircuit, resistances of a main resistor 625 and the auxiliary resistor605 a are added in parallel upon activation of the boost circuit 605such that the transconductance of the droop amplifier 210 is increasedthereby increasing gain of the droop amplifier 210.

The droop amplifier further includes an offset compensation circuit 610that compensates along with the boost circuit 605 such that the feedbackvoltage follows the reference voltage accurately. The compensationcircuit 610 includes a resistor (R_(OFF)) coupled to a current source(I_(OFF)). The offset voltage is given as I_(OFF)R_(OFF). In oneembodiment, the offset compensation circuit 610 is in a feedback path ofthe switching regulator having a negative value. In another embodiment,the offset compensation circuit 610 is in a reference path of theswitching regulator having a positive value. The offset compensationcircuit 610 is configured to follow the slew rate of the upwardtransition of the reference voltage. It is noted that the boost circuit605 and compensation circuit 610 are active only for higher referenceslew-rate settings (selectable as one of eight settings). The slew-ratesetting of reference for dynamic transition is digitized at start-up ofthe IC and programmable by the user and hence this information is usedto enable boost circuit 605 as well as offset compensation circuit 610for the reference.

In one embodiment, the transconductance of the circuit under normaloperating condition is K/R_(SET). During reference upward transition,the resistance of the auxiliary resistor 605 a along with the switchseries resistance is added in is parallel with the resistance of themain resistor 625, thereby increasing the transconductance of thecircuit. Similarly the offset compensation circuit 610 is added in thefeedback path, but disabled under normal operating condition of theswitching regulator. Several embodiments does not use any additionalcomplex circuitry for gain boosting and the same circuit for load lineimplementation can be reconfigured during upward transition of thecontrol reference such that the settling delay is reduced from the DC-DCswitching regulator. One embodiment provides a single reconfigurablecircuit that offers two different droop values, one during referencehigh transition and the other required value during rest of the timewhen the power supply is operational.Use of minimal additional circuitryhas advantages of no additional cost to the IC while seamless transitionbetween reference transition and steady-state operation is achieved.

In one embodiment, since the droop voltage is dependent on the high gainof the droop amplifier, the transconductance and hence the gain isincreased when the control reference of the power supply transitionsfrom a lower to a higher voltage. Depending on the gain enhancement, thedroop voltage is reduced to a lower value and hence the error at the endof the transition is reduced compared to the case where droop gain isnot increased during reference transition. Further, the offsetcompensation circuit 610 in the controller compensates for errorresulting from finite gain of the droop amplifier during upwardreference transition.

In the foregoing discussion, the term “connected” means at least eithera direct electrical connection between the devices connected or anindirect connection through one or more passive or active intermediarydevices. The term “circuit” means at least either a single component ora multiplicity of components, either active or passive, that areconnected together to provide a desired function. The term “signal”means at least one current, voltage, charge, data, or other signal.

The forgoing description sets forth numerous specific details to conveya thorough understanding of the invention. However, it will be apparentto one skilled in the art that the invention may be practiced withoutthese specific details. Well-known features are sometimes not describedin detail in order to avoid obscuring the invention. Other variationsand embodiments are possible in light of above teachings, and it is thusintended that the scope of invention not be limited by this DetailedDescription, but only by the following Claims.

What is claimed is:
 1. A switching regulator comprising: a droopamplifier, responsive to a reference voltage and a feedback voltage, togenerate a droop voltage; the droop amplifier comprising a boost circuitconfigurable to increase a transconductance of the droop amplifierduring an upward transition of the reference voltage; and a comparatorresponsive to the droop voltage and a current sense signal, thecomparator being configured to initiate switching in the switchingregulator.
 2. The switching regulator of claim 1, wherein the droopamplifier comprises a main resistor and an auxiliary resistor, theauxiliary resistor being coupled in parallel to the main resistor inresponse to the upward transition of the reference voltage through aswitch.
 3. The switching regulator of claim 1, wherein the boost circuitcomprises the auxiliary resistor coupled to a switch, the switch beingcontrolled to activate the boost circuit during an upward transition ofthe reference voltage.
 4. The switching regulator of claim 3, whereinthe boost circuit is activated based on an output of an up/down counterwherein the up/down counter receives a target DAC code and compares thetarget code against the current such that during the upward transitionof the reference voltage the boost circuit is activated.
 5. Theswitching regulator of claim 1, wherein the reference voltage changeswith time and wherein the reference voltage is generated from an outputof a digital to analog converter.
 6. The switching regulator of claim 3,wherein resistances of a main resistor and the auxiliary resistor andswitch are added in parallel upon activation of the boost circuit suchthat the transconductance of the droop amplifier is increased therebyincreasing gain of the droop amplifier.
 7. The switching regulator ofclaim 1 further comprising an offset compensation circuit thatcompensates along with the boost circuit such that the feedback voltagefollows the reference voltage.
 8. The switching regulator of claim 3,wherein the offset compensation circuit is in a feedback path of theswitching regulator having a negative value.
 9. The switching regulatorof claim 3, wherein the offset compensation circuit is in a referencepath of the switching regulator having a positive value.
 10. Theswitching regulator of claim 3, wherein the offset compensation circuitis configured to follow the slew rate of the upward transition of thereference voltage.
 11. The switching regulator of claim 1 furthercomprising: a plurality of switches responsive to an output of thecomparator to switch the switching regulator; an LC filter coupled to acapacitor; and a load coupled to the switching regulator.
 12. Aswitching regulator comprising: a droop amplifier, responsive to areference voltage and a feedback voltage, to generate a droop voltage;the droop amplifier comprising a boost circuit configurable to increasea transconductance of the droop amplifier during an upward transition ofthe reference voltage, wherein the droop amplifier comprises a mainresistor and an auxiliary resistor, the auxiliary resistor being coupledin parallel to the main resistor in response to the upward transition ofthe reference voltage through a switch, and wherein the boost circuitcomprises the auxiliary resistor coupled to a switch, the switch beingcontrolled to activate the boost circuit during an upward transition ofthe reference voltage; and a comparator responsive to the droop voltageand a current sense signal, the comparator being configured to initiateswitching in the switching regulator, wherein resistances of a mainresistor and the auxiliary resistor are added in parallel uponactivation of the boost circuit such that the transconductance of thedroop amplifier is increased thereby increasing gain of the droopamplifier.
 13. The switching regulator of claim 12, wherein the boostcircuit is activated based on an output of an up/down counter whereinthe up/down counter receives a target DAC code and compares the targetcode against the current such that during the upward transition of thereference voltage the boost circuit is activated.
 14. The switchingregulator of claim 12 further comprising an offset compensation circuitthat compensates along with the boost circuit such that the feedbackvoltage follows the reference voltage.
 15. The switching regulator ofclaim 14, wherein the offset compensation circuit is in a feedback pathof the switching regulator having a negative value.
 16. The switchingregulator of claim 14, wherein the offset compensation circuit is in areference path of the switching regulator having a positive value. 17.The switching regulator of claim 14, wherein the offset compensationcircuit is configured to follow the slew rate of the upward transitionof the reference voltage.